Geology and Natural History. 399 



nearly the same. No direct relation was found between the 

 radio-activity and the potential-gradient ; temperature and baro- 

 metric pressure also seemed to have no great influence. The 

 presence of clouds had a marked effect, however, the maximum 

 of 432, for a clear sky, falling to 198, when the sky was com- 

 pletely overcast. Sudden changes were occasionally noted, as 

 from the low value 66 to the high 384 within a few hours. The 

 observations were made in winter when _ the sun was below the 

 horizon and the surface of the frozen earth was covered with more 

 than two feet of snow. — Proc. Hoy. Soc, lxxiii, 209. 



10. The Optical Properties of Vitreous Silica. — The remark- 

 able properties of vitrified quartz which make it suitable for cer- 

 tain applications in optical work are remarked by Gifford and 

 Shenstone. It has a definite constant composition, unlike glass ; 

 is hardly attacked by any corrosive fumes (except F and HF) 

 and is indifferent to solvents. It is also as transparent to ultra- 

 violet radiation as quartz while it has not its double refraction. 

 Its dispersive power is sensibly greater than that of quartz, while 

 the refractive index is low, approaching that of fluorite. A 60° 

 prism, 41 mm high by 32 mm wide, was made with great care from 

 many hundreds of fine rods of vitreous silica. Another compound 

 prism, 56 mm X38 mm , was made of four distinct slabs of silica from 

 separate meltings ; when finished its performance could not be 

 distinguished from the single prism. Values of the refractive 

 indices for the latter prism are given for a series of wave-lengths 

 from 7950 to 1852*2 ; f or D (Na) the value obtained was 1-4584772. 

 For a thin doublet of fluorite achromatized by vitreous silica it 

 was found that the focal length was almost independent of wave- 

 length. — Proc. Roy. Soc, lxxiii, 201. 



1 1. Terrestrial Magnetism. — A Department of International 

 Research in Terrestrial Magnetism has been established by the 

 Trustees of the Carnegie Institution. An appropriation of 

 820,000 was made for the organization of the work and an annual 

 grant of like amount is expected for carrying it on. Dr. L. A. 

 Bauer, who now has charge of the magnetic work of the Coast 

 and Geodetic Survey, has been appointed Director of the new 

 Department. 



II. Geology: and Eatukal History. 



1. United States Geological Survey. — The following publica- 

 tions have recently been issued : 



Twenty-Fourth Annual Report, 1902-1903 ; by C. D. 

 Walcott, Director. 230 pp., 26 pis. — The amount of work under 

 direction of the Geological Survey may be judged from the size 

 of the appropriation, which last year amounted to $1,377,470. 

 For purely geologic work there was allotted $163,700. A section 

 of Petrology has been created, with Mr. Whitman Cross as 

 geologist in charge. The principles of classification and nomen- 

 clature adopted in 1889 have been revised and a new code of 



